(1) Field of the Invention
The present invention generally relates to a backlight module and particularly relates to a backlight module which is capable of interchanging polarized states of light.
(2) Description of the Related Art
Please referring to FIG. 1, FIG. 1 generally shows a cross-section view of a liquid crystal display (LCD). An LCD 10 includes a liquid crystal panel 12 and a backlight module 14. The backlight module 14 provides lights to the liquid crystal panel 12.
Liquid crystal panel 12 includes an upper polarizer 124, a color filter 121, a liquid crystal layer 122, a thin film transistor (TFT) substrate 123 and a bottom polarizer 125. The TFT substrate 123 comprises a plurality of pixel electrodes for controlling the twist angles of liquid crystal molecules 16 of the liquid crystal layer 122. Hence, transparency of a polarized light beam coming from the bottom polarizer 125 could be modulated by the liquid crystal layer 122, so as to display gray level images. Through the color filter 121, the LCD 10 gives out images with color.
Controlling the twist angles of liquid crystal molecules 16 to modulate the transparency of light is an essential technique of the related arts. As shown in FIG. 1, the oval-shaped liquid crystal molecules 16 have bilateral effect to light. Light which is going to be modulated by the liquid crystal layer 122 must be polarized in advance. Thus, the conventional LCD 10 requires one or a plurality of polarizers, e.g. upper polarizer 124 and bottom polarizer 125 shown in FIG. 1, to polarize the light from the backlight module 14. In some of other related arts, as shown in FIG. 2, a polarizer 145 may disposed within the backlight module 14 for replacing the bottom polarizer 125 shown in FIG. 1.
FIG. 2 shows a cross section view of a conventional backlight module. The backlight module 14 comprises a lamp 141, a light-guide plate 142, a reflection plate 143, a polarizer 145 and a plurality of optical films 147.
The lamp 141 is disposed at one side of the light-guide plate 142, therefore, the backlight module 14 is called an edge-light type backlight module. The lamp 141 may be a cold cathode fluorescence lamp (CCFL). The light-utilizing efficiency of the edge-light type backlight module 14 is generally less than another backlight module, a direct type backlight module (not shown), which is usually used in some larger-sized LCDs.
The light-guide plate 142 has an inclined plane 1421 at the side near the lamp 141, and there is a plurality of slot structures 1422 disposed at the bottom surface of the light-guide plate 142.
As shown in FIG. 2., density of the slot structures 1422 increases from the side adjacent to the lamp 141 to another side far away from the lamp 141. This avoids the light-guide plate 142 having particularly higher brightness adjacent to the lamp 141 than that far away from the lamp 141, and makes the light evenly emit from the light-guide plate 142. The reflection plate 143, disposed below the light-guide plate 142, reflects the light from the bottom surface of the light-guide plate 142 and enlarges the light-utilizing efficiency.
In backlight module 14, in order to enhance optical performance, the light generated from the lamp 141 passes through many devices described above to provide an uniform backlight source to the liquid crystal panel 12 shown in FIG. 1. As a result, the amount of the light provided to the liquid crystal panel 12 is usually less than 8% of the total amount of the light generated from the lamp 141. So the low light-utilizing efficiency becomes a particularly serious problem in edge-light type backlight module 14.
Among all devices in LCD 10, the polarizer 145, the upper polarizer 124 or the bottom polarizer 125 causes huge light loss. Generally, 50% of the total amount of the light is P-ray and the other 50% is S-ray. While light passes through a polarizer 145, 124 or 125, S-ray is absorbed and therefore 50% of the total amount of the light is lost.
A method of making polarizer 145, 124 or 125 and the functions of it are described below. First, specific molecules with long side-chain are disposed in a plastic film, which may be transparent. Then the plastic film is elongated by corresponding tension force. Thus, the side-chain of the molecules might be arranged alongside the direction of the tension force. The polarizer 145, 124 or 125 may absorb light, S-ray, having a polarization oriented perpendicular to the long axis of said molecules, only light, P-ray, having a polarization oriented parallel to the long axis of said molecules may be transmitted through polarizer 145, 124 or 125. Hence, 50% of light is lost when it passes through the polarizer 145, 124 or 125.
Thus, the related art lacks the way of utilizing the absorbed S-ray. Therefore, a backlight module which is capable of utilizing light more effectively is needed. Besides, considering the production line, cost and convenience of making the present provided backlight module is another important issue. As a matter of fact, all the R&D staffs in the related field of LCD development are dedicated in resolving the lacking of related art in hoping to increase the efficiency and quality of light provided by the backlight module.